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Need a Few Second Opinions

Need a Few Second Opinions

Need a Few Second Opinions

(OP)
I have a condition where a guyed tower is failing just above the torque arm, where the tower diagonals are failing in compression. I assumed a K factor of 1.0 conservatively, but wonder if I am too conservative here due to the configuration of the webbing. The attached sketch shows the web reinforcing pattern - Z bent rods, continuous and overlapped, one welded to the backside of the pipe column leg, and one to the front. A previous engineer did not fail the tower, but used a K factor of 0.7 for these members.

Fix-Fix recommended K is 0.65, Fix-Pin is 0.8, pin-pin is 1.0...

In thinking about it, due to the continuity OF THE ROD MEMBER with respect to the connection, ignoring the weld, I can see where you could reason end fixity and justify the 0.7. The fixity exists due to the continuity of the rod through the connection, not the weld to the pipe column. As the rod, being 5/8" A36 material, was probably cold bent to get the shape, then shop welded to the column, what happens to any continuity when the rod is welded to the pile column? Is some relative continuity lost or is it maintained?

This is the first time I have come across this situation and would like some feedback.

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

Left to my own devices, I would absolutely have assumed K=1 as you did. Of course, now we're in desperation/creative mode.

I buy your clever continuity argument with two qualifications:

1) The rod ends won't be fixed. They'll be provided with rotational restraint that depends on the stiffness of the "back span" tension member. I think that this should be evaluated in at least a cursory fashion.

2) You'll need to make this argument for buckling in the lateral direction as well. That will involve the torsional stiffness of the webbing. Tricky.

Got any photos of the welds?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Need a Few Second Opinions

What about a field test by point loading one of the horizontal rods until you get elastic deflection and observe the curvature of the rod, do you think that would give you an idea of the end restraint?

Maine EIT, Civil/Structural.

RE: Need a Few Second Opinions

I second the lateral direction concern of kootk.

RE: Need a Few Second Opinions

(OP)
Very good point KootK. A BIG Star for that comment. Thanks.

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

I was digging through some old steel design information for a job recently and came across a clause for truss design where they said you can assume pin-pin for all members AND your length should be considered as 0.9d where d is the distance between the panel points.

Not sure if this 10% reduction in length is what would be enough for your diagonals to work or not.

RE: Need a Few Second Opinions

(OP)
jayrod12:

That would mean an effective K of 0.9. Did they reference an AISC section or article?

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

Mike:
Are these diags. failing in the real world, or are they failing in your analysis only? Your sketch shows on face of a three legged tower right, and the two other sides are similar? And, the torque arm is attached only to this one side/face truss, and is where they guy wires connect to the tower right? Would you improve the situation by moving the guying points closer to the tower on the torque arm, a lesser induced moment? How is the tower guyed in the other two side/face planes? How do these three planes share in taking this induced moment, to lessen the moment cap’y. needs in the plane you show?

I think your fixity is partly/primarily due to the continuity of the 5/8" of the rod through the joint, with some added fixity due to the welds to the pipe columns. The weld fixity portion is a function of the weld size and quality vs. the stiffness of a 2.5' + long 5/8" dia. bar (pretty weak). However, these welds are always pretty crappy in terms of their root penetration and quality and their end terminations. They are prone to defects and stress raisers, which tend to make them failure points in an active structure.

You could cut the lengths of the diags. in half, if you stiffened the horiz. web members and then braced the diags. to them. Thus... take a 5/8" x 1.5" wide bar, 12-16" long (almost col. to col.); use a U-bolt at each end of this bar to fix it to the horiz. web 5/8" round. Then, at the crossing point btwn. the horiz. and diag. web members (rounds) you will need something like a 7/8" U-bolt to clamp both web members together, over the flat bar. Granted, this is a bit like propping three drunken sailors against each other for support, until one of them moves. On the other hand the bracing force needed is a fairly small % of the axial force involved, so maybe this would give you some added confidence that .5, .65 or .7 are more reasonable K factors, or that you have decreased the length of the compression member. And, you don’t need a welder up at 52'. The U-bolt idea is for concept only. I would probably design some hardware that fit and worked better and have a machine shop make 30 of them for me.

RE: Need a Few Second Opinions

It was an CSA S16-1969. Can't remember the exact clause and I've now filed that standard back in the basement. Although it appears to exist in the open web steel joist section of the new CSA S16-09 under clause 16.5.7.2 where it says for the horizontal axis you can use k=0.9 and Lx as the distance between panel points, use k=1 and distance between attachment to deck for the vertical axis, and for skew you can go back to k=0.9 and L as panel point spacing.

RE: Need a Few Second Opinions

although I now realize that was for chord segments. I'll see if I can track down the reference for web members.

Haha found it, 16.5.8.2, if the web consists of individual members than you can use k-0.9 but if it is built-up then k=1

RE: Need a Few Second Opinions

(OP)
dhengr:

The diagonals are failing in the computer analysis only. The tower webbing configuration is the same on all three sides, and the torque arm is a three sided structure composed of C8 members that is U-bolted to the legs of the tower at the midpoints of the C8's. The three guys are connected to the three tips of the triangular torque arm. It is a "Star Mount" configuration for the torque arm.

I would not count on the welds for any fixity as they are only about 1 to 1.5" long, curved to the bend of the rod.

I agree that in the weak direction, you could stiffen these diagonals by adding interior triangular bent rods normal to the pipe columns that could be welded to the existing diagonals. That would be one fix. Another is just to weld another rod section to the existing diagonals to increase the area in compression. We will see what the client wants to do.

Presently, I am going to remain with my original result of failing the tower.

Any additional comments are welcome.

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

(OP)
jayrod12:

This is a Canadian Code?

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

Yup sorry, don't do the american stuff. I've looked through some of them out of curiousity but never needed to use them.

RE: Need a Few Second Opinions

Msquared -

check out Table 4-5 of the TIA G standard. K can be anywhere between 1.1 and 0.7 for these members, depending on the L/r value.

I don't know exactly what criteria they used to develop that table. But, it's given right in the governing code. Therefore, I'd want a compelling reason to justify some other criteria.

RE: Need a Few Second Opinions

Seeing as the member in question is a bent rod perhaps the Steel Joist Institute could provide justification for another criteria, if needed. After all, if anyone knows about bent steel rod it should be them.

RE: Need a Few Second Opinions

(OP)
Josh:

I looked at the table, and, logically it makes no sense to me, code or otherwise. I am wondering if this is an error in the code.

To me, since the K value decreases as the L/r ratio increases, the table is saying that more fixity is generated with higher L/r values, which is contrary to my logic. Until I understand this, I will stick with the 1.0. I just do not believe it.

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

I don't have this table but the reported results make some intuitive sense to me.

You're never truly fixed. Every boundary condition is a rotational spring of some kind. So you've really got two buckling modes to consider:

1) A buckling mode where the springs give out early and the column buckles ~K=1.0 style.
2) A buckling mode where the springs stay in the game and the combined system buckles ~K=0.7 style.

Which mode governs will depend on the ratio of the column stiffness to the rotational spring stiffness. Thus, for a given end restraint stiffness, a more slender column means a K value closer to 0.7.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Need a Few Second Opinions

Mike:
I’m not sure which is the weak direction when we are talking about a 2.5" + long x 5/8" round bar in compression. But, I claim my 5/8" x 1.5" flat bar, from col. to col. in length, on the horiz. web member, would brace the 5/8" round diag. web member in two directions, both in the plane of that side/face truss and perpendicular to that plane, thus cutting the compression member length in half. You won’t get any argument from me about the dubious quality of the original welds on those kinds of details, but they are holding the tower together, aren’t they? I would not attribute greater weld quality and integrity to welds being made 50' in the air, on a windy day, by some guy hanging off that tower, hung-over or not.

I don’t have the code JoshP refers to, so I can’t study the table in question. But, given what the two of you have said, isn’t the issue that as the L/r of a member increases (it becomes less stiff w.r.t. buckling), any amount of fixity becomes more significant in how the compression member acts w.r.t. that fixity. i.e. any form of fixity, say 100 ft.lbs. of rotational resistance, by some means, maybe inferior welds, is not very significant when the col. is a W14 section; but that same 100 ft.lbs. of restraint is pretty significant w.r.t. your 5/8" round by 2.5' + long compression member. Thus, the locations of the inflection points change on the fixed-fixed col. and/or lead to variations in the magnitude of the K values.

RE: Need a Few Second Opinions

(OP)
KootK:

The table I posted in a ink in my last post.

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

(OP)
KootK:

I think you are inadvertently mixing apples and oranges here.

The table I included above is for the COMPRESSIVE capacity of the round DIAGONALS only by generating the effective slenderness ratios (the diagonals are currently failing in compression, not tension) and has absolutely nothing to do with the size of the column legs. The column legs are just fine and do not even figure into the table at all.

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

No, I'm still on point. Does it make more sense like this:

Quote (KootK)

I don't have this table but the reported results make some intuitive sense to me.

You're never truly fixed. Every boundary condition is a rotational spring of some kind. So you've really got two buckling modes to consider:

1) A buckling mode where the springs give out early and the column round diagonal buckles ~K=1.0 style.
2) A buckling mode where the springs stay in the game and the combined system buckles ~K=0.7 style.

Which mode governs will depend on the ratio of the column round diagonals stiffness to the rotational spring stiffness. Thus, for a given end restraint stiffness, a more slender column round diagonals means a K value closer to 0.7.

It was my version of attempting to say the same thing as this:

Quote (dhengr)

isn’t the issue that as the L/r of a member increases (it becomes less stiff w.r.t. buckling), any amount of fixity becomes more significant in how the compression member acts w.r.t. that fixity.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Need a Few Second Opinions

I want to say that I saw a presentation once that pointed to an ASCE publication where those formulas were derived. But, I don't have an exact reference. I can say that CSA table 7 has similar (though not identical) formulas.

It makes intuitive sense to me for the same reasons that KootK suggests. It's like the the G table for AISC columns. If you keep the beams the same, but use more and more slender columns, the K value you calculate for the column goes down.

RE: Need a Few Second Opinions

(OP)
OK. I can see that... I can see that only in the plane of the bracing itself, and the buckling of the rod transverse to that. However, these are relatively different conditions...

In the plane of the bracing, both the weld and the continuity of the member contribute to the stiffness, while in the transverse direction, only the small weld does. That is to say that the in plane fixity is more significant than the out of plane fixity. So it looks to me like the 0.7 flies for the in plane buckling condition, perhaps 0.9 for the out of plane condition based on jayrod12's post, giving 0.9 as governing overall for the diagonal elements.

Comments?

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

Are the rods failing under compressive (buckling) loads near the middle of the rods?

Or are the rods failing at the ends as if they were strong enough from end-t-end but need a more rigid end point attachment to the tower verticals?

RE: Need a Few Second Opinions

(OP)
Computer program does not tell me that. Only that certain members are are failing between certain nodes. This is TnxTower...

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

Blast.

OK. Try this: Do a mass substitution of all of skinny rods into 1 inch diameter rods for the failing members. Don't change the end conditions or the end welds. See if they still fail.

RE: Need a Few Second Opinions

(OP)
I already did at 3/4" and they did not fail.

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

I know you didn't ask but I couldn't resist trying my hand at a weld free repair detail. I've seen a similar strategy employed as buckling prevention for raised floor support rods.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Need a Few Second Opinions

Mike...check the lateral movement of the vertical members...might be enough to throw your fixity conditions off at the diagonals.

RE: Need a Few Second Opinions

(OP)
Ron:

The program takes into account P Delta, so that really should have been considered already. Perhaps it is a global P Delta, though, and not a local...

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

(OP)
KootK:

That would work too with just another 5/8" rod.

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

I suppose that it would, wouldn't it...

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Need a Few Second Opinions

That's a lot of nuts and bolts to double-up every cross-rod. On the tower, there would be about 1 rod very 30 inches x 3x sides x height of tower, right?

Why not add one long vertical angle iron (or channel) across every cross-rod at the middle of the cross-rod? Bolt it in place to hold it up, then weld each cross-rod to the angle iron (or channel)? The weld supports the middle of the slender cross-rod and ties it both up and down to the next higher and next lower cross-rod, and the back leg of the angle iron (both back legs of the channel) stiffen the cross-rods against motion in-and-out compared to the center of the triangular tower.

A single angle iron 20 feet long would stiffen every rod it is welded to.

RE: Need a Few Second Opinions

(OP)
There are only about 15 diagonals just above the torque arm that are of concern. The rest are ok.

Mike McCann, PE, SE (WA)


RE: Need a Few Second Opinions

Doesn't make sense: Seems like it would be the rods right under the brace that would be more highly stressed? (The ones immediately above are stressed too of course, but slightly less if only by the absence of weight as you go down, and the added lever arm of the longer distance to the top.)

RE: Need a Few Second Opinions

(OP)
There are antennas, mounts, and wind loading that drive the compressive forces in the diagonals above the torque arm. The guy forces are transmitted through the torque arm directly to the three legs avoiding the diagonals below.

Mike McCann, PE, SE (WA)


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